1. Field of the Invention
This invention pertains to an energy efficient method of removing ethane and higher boiling hydrocarbons from a high pressure stream of natural gas.
2. Description of the Prior Art
Natural gas from oil and gas wells is typically produced as a gaseous mixture of methane, ethane, and higher boiling hydrocarbons, but it is primarily methane. Natural gas can also contain water, hydrogen sulfide, and other gaseous or entrained components. All of the chemical components of natural gas have value as does the chemical gemisch. However, some of the components or component mixtures are more valuable than others. E.g., sweetened natural gas is more valuable than sour natural gas because the toxic/noxious hydrogen sulfide has been removed and the health/environmental problems associated with hydrogen sulfide have been reduced or eliminated.
Natural gas is primarily marketed and used as a gaseous hydrocarbon fuel, and it is conveyed from the well to the market by a network of pipelines and storage facilities. Unfortunately, some of the hydrocarbon components and/or moisture in natural gas condense under certain conditions of temperature and pressure as the gas is transported through pipelines or stored. The presence of water in gas may cause hydrate formation with the resultant precipitation of solids which can plug lines and valves. Water condensed from natural gas may also increase corrosion of pipelines through which the gas is transmitted if the gas contains carbon dioxide or hydrocarbon sulfide.
In addition, the concentration of higher boiling point hydrocarbons, especially propane and butane, sometimes is high enough to cause condensation of liquid hydrocarbons at the high pressures in the pipeline. The liquid can collect in low spots and cause slugging through the pipeline which interferes with the transmission of the gas. To avoid condensation of water or hydrocarbons, natural gas pipeline companies specify maximum moisture and hydrocarbon dew points for gas that they purchase.
To reduce the dew points of natural gas delivered to pipelines, the gas is frequently treated at gathering points to remove moisture and hydrocarbons that may condense before transmission through pipelines. One conventional treatment at natural gasoline plants passes the gas through an absorption tower in contact with an absorbent oil which removes higher boiling point hydrocarbons from the gaseous stream. The rich oil is then passed through a stripper where the volatile hydrocarbons are removed from the oil. The absorbent oil is recycled through the absorber and the stripped hydrocarbons are delivered to a fractionating system for separation of the hydrocarbons to produce a liquid product having a vapor pressure allowing it to be safely stored in LPG vessels. More recently, natural gas plants have used a turboexpander refrigeration system for the separation of higher boiling point hydrocarbons from methanes. A natural gasoline plant of either type is expensive and cannot be justified at many small fields.
Patents disclosing the treatment of natural gas to separate methane from other constituents of the natural gas are: U.S. Pat. Nos. 2,134,702; 3,285,719; 3,292,380; 3,494,751; 3,596,472; 4,128,410. The last patent in this nonexhaustive list is particularly concerned with the removal of water from natural gas, as is U.S. Pat. No. 4,522,636 which adds methanol to the natural gas before treatment. The disclosure of U.S. Pat. No. 4,522,636 is hereby incorporated by reference. Other techniques have been described which attempt to convert natural gas to "pipeline gas" or "pipeline quality gas," as it is typically referred to in the industry. This invention is also direct to a method of obtaining pipeline quality gas.